1. Field of the Invention
This invention is in the field of screening devices for preventing ferromagnetic or magnetic metal objects from being in the vicinity of an operating magnetic resonance imaging apparatus.
2. Background Art
Magnetic resonance imaging (MRI) has been called “the most important development in medical diagnosis since the discovery of the x-ray” 100 years ago. Magnetic resonance imaging has significant risks, however, and these are becoming more apparent as the number of MRI procedures increases dramatically. In particular, ferromagnetic objects are drawn toward the magnetic resonance imaging magnet by the strong magnetic field of the magnet, sometimes with catastrophic results. This attraction of ferromagnetic objects to the MRI magnet is termed the “missile threat.” Not only have there been numerous injuries to patients, including one tragic death, but damage to the MRI magnet itself is also a significant problem.
In an effort to provide safety, MRI centers have attempted to utilize conventional metal detectors, such as those used for airport and other security applications. Conventional metal detectors alarm not only on ferromagnetic threat objects, but also on non-threat, non-ferromagnetic, metallic objects. The huge number of false positive alarms generated by conventional metal detectors has caused such consternation for MRI staff technicians that conventional metal detectors have been abandoned for this application. Indeed, conventional metal detectors may have no current usefulness as a practical solution for MRI safety.
A ferromagnetic object, such as a small pipe wrench, can be drawn in instantaneous missile-like fashion toward the MRI magnet. The force of the magnetic attraction between the pipe wrench and the MRI magnet causes the wrench to fly toward the magnet as if propelled by a rocket. A tragic death has occurred from the missile threat, and many “near misses” have occurred. One MRI disaster involved a bobby pin being impaled in the nasal passages of a patient, requiring surgical extirpation.
Magnifying the threat potential for serious harm is the next generation of MRI magnets, which are even more powerful than current generations, generating magnetic fields of 3.0 Tesla, or 30,000 Gauss, as opposed to today's “standard” of 1.5 Tesla, or 15,000 Gauss.
Magnetic Resonance Imaging (MRI) scanners utilize extremely high magnetic fields. It is well known that very serious accidents can occur if ferromagnetic objects are carried into the vicinity of an MRI scanner. To minimize this danger, ferromagnetic-detecting portal inventions and ferromagnetic-detecting wand inventions have been developed to detect ferromagnetic threat objects in the magnetic resonance imaging environment, and so minimize the chance of dangerous accidents.
Ordinary ferromagnetic objects are only magnetic if a magnetic field is applied. A portal can create such a magnetic field by using current flowing in coils or by using permanent magnets. This applied field magnetizes the ferromagnetic threat, which can then be detected by the sensor system of the portal. The fringe field of the MRI magnet can also be used as the applied field.
Another type of portal is “passive,” meaning that it has no independent applied magnetic field source, but rather depends upon the 0.5 Oersted (Oe) field of the earth for magnetization of a ferromagnetic threat object. This small field is generally adequate for ordinary “soft” iron or steel objects, like a carpenter's nail, which are easily magnetized.
However, some common ferromagnetic objects are made of tempered spring steel. The ordinary “bobby pin” used in women's hair is a good example. The magnetic properties of such materials make them hard to magnetize and they are, therefore, called “magnetically hard.” FIG. 1 below shows the initial magnetization curve of a typical bobby pin.
As can be seen from FIG. 1, an applied field higher than 100 Oe is required to achieve a magnetization of 98%, and a field higher than 50 Oe is needed to achieve a magnetization of 80%. The applied field in a 1.5 Tesla MRI apparatus is 15,000 Oe, which is sufficient to create a missile threat with even a magnetically hard object. So, tempered steel or other magnetically hard objects like bobby pins are just as dangerous as ordinary soft steel items, from the missile threat point of view, and it is important that these be detected.
Some types of magnetic fields are insufficient to magnetize certain ferromagnetic threat objects, including bobby pins and the like, for detection by a ferromagnetic-detecting portal or a ferromagnetic-detecting wand. For example, in the very small 0.5 Oe magnetic field of the earth, the magnetization of the bobby pin is virtually zero, as shown by the arrow in FIG. 1. In addition, the earth's magnetic field for a particular location on the planet is in only one axis, and detection can be missed when the major axis of a ferromagnetic threat is perpendicular to the earth's magnetic field. This means that a bobby pin would be very poorly detected, if at all, by a passive portal that depended on the earth's field to magnetize the objects being detected.
As a further example, the magnetic fringing field of the MRI magnet is generally only 1 to 5 Oe outside the MRI magnet room itself. In addition, the MRI fringing field does not apply magnetization in all three axes, and, if the threat object is perpendicular to the fringing magnetic field, detection is often compromised because of insufficient magnetization.
As a final example, the permanent magnets or coil arrangements of a strength typically found on an “active” portal or wand may be insufficient to magnetize certain difficult-to-magnetize ferromagnetic objects, such as bobby pins. Also, the excitation magnetic field associated with a portal with its own applied magnetic field system is often applied in less than all three (x, y, and z) axes, since it is impossible to apply a magnetic field in all three axes when the field source and the detection point are stationary. This can result in insufficient magnetization, especially of difficult-to-magnetize ferromagnetic threat objects. For instance, if an applied field is only applied to provide detection of a threat for the x axis, it is possible that a threat may escape detection because of insufficient magnetization for the y axis and the z axis. The inability to apply the magnetic field in all three axes can be partially overcome, however, if the applied magnetic field strength in one axis is sufficiently great to induce magnetization sufficient to allow detection. Further, the chance of detecting a threat object in such a portal is increased if the subject is asked to rotate or tilt within the portal pass-through aperture.
There may be additional reasons for insufficient magnetization to support detection of a threat object. The object may be tiny, or the configuration of the object may make magnetization, and hence, detection, difficult. For instance, a round object is more difficult to magnetize and detect than an elongated one. Further, the ferromagnetic threat object may be located toward the midline of the portal's pass-through aperture, such as in the middle of a 32 inch portal aperture, or 16 inches away from the sides of the portal. Or, the threat object may be located at too great a distance from a ferromagnetic-detecting wand, such as when the wand is waved at too great a distance from the threat object to achieve the required magnetization.
In any of the above instances, as well as others, the result is insufficient magnetization to allow the sensing system of the portal or wand apparatus to detect the threat, resulting in a false negative (missed) response. Then, when the patient enters the magnetic resonance magnet room, disaster can strike, as the ferromagnetic threat now becomes completely magnetized by the considerably more powerful MRI magnet and, when the critical point is reached in relation to the distance to the MRI magnet, the threat object is propelled toward the magnet as if propelled by a rocket.